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Field emission of graphene oxide decorated ZnO nanorods grown on Fe alloy substrates
Journal of Alloys and Compounds 729 (2017) 538e544
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Field emission of graphene oxide decorated ZnO nanorods grown on Fe alloy substrates Jijun Ding a, b, *, Haixia Chen a, Li Ma b, Haiwei Fu a, Xiaojun Wang b, ** a b
College of Science, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2017 Received in revised form 19 September 2017 Accepted 20 September 2017 Available online 21 September 2017
Graphene oxides (GO) are decorated on the top surface of ZnO nanorods (NRs) grown on Fe alloy substrates for efficient field emission. The GO decorated ZnO NRs acting as cold electron emitters exhibit excellent field emission performance with the turn-on field Eto as low as 1.63 V/mm and the threshold field Ethr down to 3.12 V/mm. ZnO NRs grown on the alloy substrates have a low interfacial resistance and intend to enhance electrical conduction. A schottky contact of Fe-ZnO and matched Fermi levels of ZnO-GO interface contribute to the enhanced current emission efficiency. Besides, some nanometer-scaled sharp protrusions have been formed in the GO sheets, and GO itself owns abundant C-O-C oxygen functional groups that also help to improve the field emission current. A straight line Fowler-Nordheim plot of the field emission current from the emitter is obtained and the effective work function for the decorated GO sheets is calculated from the slope with a value below 1.5 eV. Finally, field emission mechanism of the GO decorated ZnO NRs has been proposed. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. © 2017 Elsevier B.V. All rights reserved.
Keywords: Graphene oxides ZnO nanorods Field emission Emission mechanism
1. Introduction ZnO is an ideal material for optoelectronic devices due to its wide band gap (~3.37 eV) and large exciton binding energy (~60 meV) [1,2]. It can be applied to ultraviolet lasers, transparent conductive contacts, laser diodes, solar cells, thin film transistors and emitters [3,4]. One-dimension (1D) nanostructures such as ZnO nanorods (NRs), ZnO nanowires (NWs), and ZnO nanocones have attracted considerable interest due to their novel properties and applications. Among them, field emission properties of various 1D ZnO nanostructures have been explored extensively. Zhang et al. [5] reported that field emission properties have a close relationship with the surface morphology of ZnO nanoneedle arrays. Chiu et al. [6] concluded that the turn-on field of ZnO NWs can be significantly decreased after Ga doping but is insensitive to the contents of Ga doping. Li et al. [7] fabricated the ZnO NWs by tuning density with
* Corresponding author. College of Science, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China. ** Corresponding author. E-mail addresses: [email protected] (J. Ding), [email protected] (X. Wang). https://doi.org/10.1016/j.jallcom.2017.09.216 0925-8388/© 2017 Elsevier B.V. All rights reserved.
the thickness of the Al film and displayed excellent field emission performance. To further improve the field emission performance from nanostructured ZnO emitters, different methods are reported, such as doping with metals [8,9], optimizing their shape and aspect ratio [10], decorating with Au nanoparticles [11], exposing to UV [12] and coating surface with low work function materials for low turn on field [13]. On the other hand, composite nanostructures of ZnO and graphene or graphene oxide (GO), owing to excellent field emission, have already been highlighted [14,15]. Graphene as a twodimensional macromolecular sheet of carbon atoms has superior electrical conductivity and mechanical properties [16], making it an excellent electron-transport material [17]. Zhang et al. [18] investigated electronic structures and field emission properties of hybrid graphene-ZnO by density functional theory and showed that the graphene-ZnO is a promising candidate for a field emission electron source. Hwang et al. [19] reported that ZnO NWs/graphene hybrids exhibit excellent field emission properties owing to the mechanical flexibility, transparency and low contact barrier. Recently, ZnO NW tips are coated by graphene dispersion to increase surface volume ratio for further improving field emission [20], however, graphene did not inherit the nanotip geometry since
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graphene is in debris segment after spin coating and plasma etching. Interestingly, Ye et al. [21] found that high density protrusions can be localized in a large area and thus enhance the electric field when GO sheets are coated on the top surface of the Ni array nanotips. Until now, there have been few attempts of surface decorating GO on alloy substrate semiconductor ZnO NRs and concerning their electron emission characteristics and the emission mechanisms associated with the GO's low dimensionality. Herein, we demonstrate a novel nanostructures that may find potential applications in high performance field emitters considering a synergistic effect of alloy substrate (enormous free electrons), ZnO NRs (high aspect ratio) and GO (numerous sharp edges and oxygen functional groups). In addition, the semiconductor nanostructures deposited on alloy substrates provide important technological merits such as the low interfacial resistance and intend to enhance electrical conduction in field emitters, and thus improve current emission efficiency. In this paper, based on GO with unique chemical structures that are easier to form excellent contact with semiconductor ZnO than graphene, combining the superior properties of GO with high aspect ratio of 1D ZnO NRs, GO decorated ZnO NRs have been grown on Fe alloy substrates. Unlike graphene being coated on the top of the emitter and shaped as the emission tips in previous literature, in our work, thin even GO sheets with abundant C-O-C oxygen functional groups are designed to decorate on ZnO NRs. Results indicate that this not only provides another channel for efficient improving field emission due to the matched Fermi levels between ZnO and GO interface, but also further decreases the potential barrier during the field emission process due to a low effective work function of the decorated GO sheets. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. 2. Experimental section Metal alloy substrate containing mainly iron (99 at.%) are cut, ground and polished, then ultrasonically clean with acetone and alcohol in sequence for 15 min. Subsequently, argon plasma clean for 10 min before growing ZnO NRs. ZnO NRs coated with GO sheets are fabricated by the following procedures. Firstly, ZnO seed layers are deposited onto the cleaned metal Fe alloy substrates by the conventional sol-gel and spin-coating technique and annealed at 350 C for 1 h in air. Secondly, ZnO NRs are synthesized by chemical bath deposition (CBD) method. The ZnO seeds are immersed in a 100 ml mixed aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 25 mM) and hexamethylenetetr-amine (C6H12N6, 25 mM) at 95 C for 6 h, and the surface are polished with the seeds side facing downward. After growth, the samples are washed by deionized water and are dried by nitrogen gas. The similarly fabricated processes about ZnO NRs have been reported elsewhere [22]. In addition, the GO is prepared from natural graphite (Wodetai Ltd. Co., Beijing, China, 99.9%) by a classical Hummers method with some modification [23]. The GO brilliant-yellow mixture is filtered and washed with 10 wt.% HCl aqueous solution (1000 ml) to remove metal ions and wash repeatedly with H2O to remove the acid until the pH of the filtrate is neutral. The resulting GO slurry is dried in a vacuum oven at 60 C. Typically, 50 mg of the as-prepared GO powers are dispersed into 100 ml of water under mild ultrasound for 1 h. Then, the GO solvents are centrifuged with 4800 rpm/min to obtain even and stabilized GO with transparent yellow-brown suspension. Finally, GO solvents with good dispersions are used directly as electrolyte. The coated GO sheets on the down-layer ZnO NRs is achieved by electrodeposition. Fe metal alloy substrate with as grown ZnO NRs is acted as working electrode and
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platinum plate is cathode that is kept 10 mm away from the counter electrode, a constant current of 0.10 mA is applied for 20 min to coat GO on the down-layer ZnO NRs. The sample is dried in nitrogen before characterization of various properties. X-ray diffraction (XRD) patterns of the bare ZnO NRs and GO decorated ZnO NRs are studied using a D/Max-2400 X-ray diffractometer. The scanning electron microscopy (SEM) images of samples are characterized by a FEI Quanta 250 scanning electron microscope. Raman spectra are investigated using a JY LabRAM HR800 laser Raman spectrometer from 200 to 3000 cm1 at room temperature. The 633 nm line of the laser is used as the excitation source. The current density-field strength (J-E) characteristics are estimated under base pressure 105 Pa at RT using a computercontrolled power source with amperometer (Keithley, 248). A glass plate with ITO is used as the anode and the samples are served as the cathode. The distance between the cathode and the anode is kept 300 mm, which is adjusted with insulating polyethylene film with a high breakdown voltage and certain thickness before the measurements. All spectra are measured at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of bare ZnO NRs and GO decorated ZnO NRs grown on Fe alloy substrates. ZnO (100), (002), (101), (102) and (103) diffraction peaks and Fe alloy substrate phase are observed in the XRD patterns. As GO sheets are decorated on ZnO NRs, ZnO (002) diffraction peak intensity increases. For comparison, ZnO and Fe diffraction peaks from JCPDS standard cards are drawn on the bottom of Fig. 1. In addition, as prepared GO has a diffraction peak centered at 2q ¼ 10.8 corresponding to a GO (002) phase. It is theoretically expected to be observed in the XRD pattern. However, the GO peaks are not clearly seen in the XRD patterns due to the small amount of GO in the sample compared with ZnO, which is similar with the previous report [24]. Therefore, it can be seen that the diffraction peaks are mainly from the ZnO and Fe alloy substrate phase in the GO decorated ZnO NRs. According to the Bragg formula: l ¼ 2dhklsinq, where dhkl denotes the crystalline plane distance for indices (hkl). It is found that all d(002) values, which are 0.2596 and 0.2590 nm for bare ZnO NRs and GO decorated ZnO NRs, respectively, are smaller than that of standard ZnO d0 value of 0.2603 nm. The lattice constants c of (002)
Fig. 1. XRD patterns of bare ZnO NRs (middle) and GO decorated ZnO NRs (top) grown on Fe alloy substrates. The ZnO and Fe diffraction peaks from JCPDS standard cards are drawn on the bottom by black and magenta lines, respectively.
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peak can also be calculated by the following formula [25]:
dhkl
2
" #1 4 h2 þ k2 þ hk l2 ¼ þ 2 : c 3a2
(1)
where both parameters a and c are the lattice constants. According to Eq. (1), the lattice constant c is equal to 2d for the (002) diffraction peak. Fig. 2 presents SEM images of bare ZnO NRs (a-b), GO decorated ZnO NRs (c-e) with different magnifications. As shown in Fig. 2aeb, the ZnO NRs with hexagonal structures are presented in the SEM image of bare ZnO NRs. The ZnO NRs are approximately same in tall and the diameter of ZnO NRs is even. After GO decorating, the ZnO NRs are almost fully covered with thin transparent GO layers, as Fig. 2cee depicts. Based on the SEM observation shown in Fig. 2e, the corresponding diameter distribution of ZnO NRs and average
area of GO sheets is shown in Fig. 2f. The average diameter of ZnO NRs and the average area of GO sheets are about 80 nm and 3.6 mm2 calculated using the particle size statistics distribution software. In order to further investigate the crystalline quality, structural disorder, and defects in samples, Raman spectra of samples are characterized, which is generally considered to be the most powerful nondestructive technique [26]. As shown in Fig. 3, a peak centered at 553 cm1 associating with oxygen deficiency in the bare ZnO NRs is observed, which is also reported by Xu et al. [27]. As GO sheets are decorated on ZnO NRs, the intensity of the peak at 553 cm1 decreases. At the same time, both sharp peaks at 1327.5 and 1591.9 cm1 corresponding to the D and G peaks of GO sheets are observed, respectively. The D peak is due to the presence of structural disorders in GO sheets. The G peak is attributed to the optical E2g phonons at the Brillouin zone center, whereas the ratio of the intensity of the G-band to the D-band is related to the inplane crystallite size [28]. Besides, the intensity ratio of IG/ID is
Fig. 2. SEM images of bare ZnO NRs (aeb) and GO decorated ZnO NRs (cee) with different magnifications. The ZnO NRs with hexagonal structures are presented in the SEM image of bare ZnO NRs. After GO decorating, the ZnO NRs are almost fully covered with thin transparent GO layers. The average diameter distribution of ZnO NRs (columnar bar) and the average area of GO sheets ((dotted line)) are shown in (f).
J. Ding et al. / Journal of Alloys and Compounds 729 (2017) 538e544
! 3 J Ab2 BF2 1 ln 2 ¼ ln , ; F b E E
541
(3)
3
6:83 103 F2 cm1 : K
b¼
(4)
where J is in the unit of mA/cm2, E in V/mm, and F the work function of the emitter, that is 5.3 eV for ZnO. A ¼ 1:54 106 A,eV,V 2 , 3 B ¼ 6:83 109 eV 2 ,V,m1 , and b is the field emission enhancement factor that is introduced to quantify the enhancement degree of any tip over a flat surface, i.e., b represents the true value of the electric field at the tip compared to its average macroscopic value [31]. Fig. 5 illustrates F-N plots of bare ZnO NRs and GO decorated ZnO NRs grown on Fe alloy substrates. The F-N plot of the field emission current from the bare ZnO NRs displays a nonlinear relation between ln(J/E2) and 1/E. It is only at very high electric fields that the F-N plot exhibits linear behavior. This nonlinearity can be ascribed to the nonuniform geometries of the emitters [21]. The estimated field enhancement factor b is 1268 cm1 calculated from the slope K of ln(J/E2)-1/E plots. As GO sheets are decorated on ZnO NRs, the plot is a straight line for nearly all measured fields, indicating that the field emission process is a barrier tunneling, i.e., a quantum mechanical process [32]. It is assumed that the local b of the sharp GO protrusions is similar to that of the ZnO NRs. Using the slope, we calculate an effective work function for the decorated GO sheets with a value below 1.5 eV. The reduction of the work function of GO sheets may be due to the sharp bend at the protrusions, and similar result is reported in the GO coated Ni nanoarrays and an effective work function of GO is below 2 eV [21]. All these results indicate that the field emission of ZnO NRs is improved largely by surface decorating GO sheets. Fig. 6 depicts the field emission current stability of GO decorated ZnO NRs for over 2 h at the distance of 300 mm from sample surface to anode under an applied constant voltage of 1425 V. The initial current density is at 0.70 mA/cm2 and the mean value Jmean is about 0.69 mA/cm2. We evaluate the field emission stability of the GO =
Fig. 3. Raman spectra of bare ZnO NRs (lower line) and GO decorated ZnO NRs (upper line) grown on Fe alloy substrates. Only a peak centered at 553 cm1 associating with oxygen deficiency is observed in the bare ZnO NRs. As GO sheets are decorated on ZnO NRs, the intensity of the peak at 553 cm1 decreases. At the same time, both sharp peaks at 1327.5 and 1591.9 cm1 corresponding to the D and G peaks of GO sheets are observed, respectively.
widely used to characterize the defect quantity in graphene and a low ratio indicates a great disorder arising from structural defects [29], which is consistent with XRD results. Fig. 4 displays field emission J-E characteristics of ZnO NRs and GO decorated ZnO NRs grown on Fe alloy substrates. In order to better compare with GO decorated ZnO NRs, the current density intensity of ZnO NRs is expanded 12 times. The turn-on field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. The Eto value of ZnO NRs and GO decorated ZnO NRs are 3.37 and 1.63 V/mm, and Ethr are 14.00 and 3.12 V/mm, respectively. The field emission J-E characteristics can be expressed by a simplified Fowler-Nordheim (F-N) equation [30]:
J¼
Ab2 E2
F
exp
! 3 BF2 : bE
(2)
The formula can be changed as following:
Fig. 4. Field emission J-E characteristics of bare ZnO NRs (solid squared line and its 12times enlargement for better comparison) and GO decorated ZnO NRs grown on Fe alloy substrates (dotted line). The turn-on field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. Both values largely decrease due to the surface decorating of GO sheets, indicating the improvement of field emission characteristics.
Fig. 5. F-N characteristics of bare ZnO NRs (solid squared line) and GO decorated ZnO NRs (dotted line) deposited on Fe alloy substrates. In the case of the bare ZnO NRs, it is only at very high electric fields that the F-N plot exhibits linear behavior. As GO sheets are decorated on ZnO NRs, the plot becomes a straight line for nearly all measured fields. It is assumed that the local b of the sharp GO protrusions is similar to that of the ZnO NRs. Using the straight line slope, we calculate the effective work function for the decorated GO sheets with a value below 1.5 eV, indicating that the potential barrier of the electrons decreases, making the emission efficient.
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Fig. 6. Field emission current stability characteristics of GO decorated ZnO NRs for over 2 h under an applied constant voltage of 1425 V. The wine solid line shows the linear fitted result and the Jdrop is 1.79% during the test period. The inset is a uniform surface emission image of GO decorated ZnO NRs. The diameter of the emission image is about 0.75 cm.
decorated ZnO NRs using a parameter Jdrop. It is calculated from (Jfirst-Jlast)/Jfirst, where Jfirst and Jlast are the first and the last J values, respectively. The Jdrop value of the GO decorated ZnO NRs is as low as 1.79%. The light drop of the emitted current is due to fieldinduced gas molecule adsorption and desorption from sample surface in the vacuum chamber. There are two other reasons that affect the stability of field emission including both vacuum condition and migration effects for different emission sites. Combined with low both turn on field and the threshold field. GO decorated ZnO NRs produce excellent field emitters, which is critical for practically field emission applications. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. The inset presents a uniform surface emission image of GO decorated ZnO NRs. The diameter of the emission image is about 0.75 cm. The mechanism of electron emission can be explained as following. ZnO NRs without coating GO have higher turn-on and the threshold electric field, which is due to the significant screening of the electric field as a consequence of the areal density of ZnO NRs. This decreases field emission current. After surface decorating
GO sheets, their field emission is improved largely. Fig. 7a presents the schematic drawing of the GO decorated ZnO NRs structures and field emission measurement at an external electric field. A glass plate with ITO is used as the anode and the samples are served as the cathode. The distance between the cathode and the anode is kept 300 mm. We believe that field emission of the GO decorated ZnO NRs structures can take place in the following three possible ways. (1) As can been seen from Fig. 7a, the electrons are transport from the bottom Fe alloy substrates to ZnO NRs and then perpendicular tunneling emission on the top surface of GO. Fig. 7b shows schematic diagram of the corresponding band scheme in the GO decorated ZnO NRs grown on Fe substrate. In our work, ZnO NRs grown on Fe alloy substrates have a low interfacial resistance and intend to enhance electrical conduction. Meanwhile, it is known that when a metal is brought in contact with semiconductor, it induces band bending due to the equilibrium of Fermi level [33]. According to previous conclusion [34,35], a schottky contact is formed at the interface region and a downward band bending is expected since the work function of metal Fe (4.5 eV) is smaller than semiconductor ZnO (5.3 eV). So electrons are easily transferred from the Fermi level of Fe alloy substrates to the top ZnO NRs. At a very low electric field, these electrons are excited from the Fe Fermi level to the ZnO conduction band. The Fermi levels between the ZnO and GO interface are matched through the generation of a space charge region, such as the p-n junction, which have been confirmed in GO coated multiple face contact junctions ZnO NRs [15]. There have also been reported that p-n junction is formed between GO and other semiconductor interface, for example in the GO/TiO2 composites [36]. Due to the electron affinity of GO is lower than that of ZnO [37], which drives the excited electrons from the ZnO conduction band to the surface of GO through ZnO-C bonding interactions. At an external electric field, these electron transport processes are accelerated. Besides, as above mentioned, according to the calculated results based on F-N plots, the effective work function for the decorated GO sheets is below 1.5 eV. Both the potential barrier height and width for the electron emission are further reduced. This further decreases the potential barrier of the electrons and makes the emission efficient. So electrons can easily tunnel through the full barrier width, causing a larger field emission current in GO decorated ZnO NRs. (2) Compared with bare ZnO NRs, both the number of macroscopic emission sites decreases and field screening effect gets weaker in GO decorated ZnO NRs. However, some nanometer-scale sharp protrusions are formed in the GO sheets as they are decorated on the top surface of ZnO NRs.
Fig. 7. (a) Schematic diagram of the GO decorated ZnO NRs structures and field emission measurement at an external electric field. The electrons are transported from the bottom Fe alloy substrates to ZnO NRs and realized perpendicular tunneling emission on the top surface of GO. A glass plate with ITO is used as the anode and the samples are served as the cathode. (b) Schematic diagram of the energy band scheme and electron transport in the GO decorated ZnO NRs on Fe substrate, and electron tunneling through the full barrier width. A schottky contact and a downward band bending are formed at the interface region between Fe and ZnO. The Fermi levels between the ZnO and GO interface are matched through the generation of a space charge region, such as the p-n junction. (EVAC: vacuum level, EV: the top of the valence band, EC: the bottom of the conduction band, EF: Fermi level, J(E): the emission current density, F: work function).
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These high density protrusions enhance the local electric field of the emitter tips and cause the perpendicular emission from 2D GO surfaces associated with its low dimensionality. The perpendicular emission has been confirmed in the individual graphene nanoribbons with high emission density [38]. (3) There is another enhanced origin of field emission needed to be considered. GO itself owns a large number of C-O-C oxygen functional groups existing in its 2D surface and edge structures, according to previous report [39], during the field emission process, the presence of C-O-C groups at its edge structures significantly reduces the barrier for electron emission into vacuum. In summary, the electron emission from GO decorated ZnO NRs is attributed to a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-GO interface, the formation of some nanometer-scale sharp protrusions in the GO sheets and the C-O-C oxygen functional groups in the GO. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. 4. Conclusions In conclusion, GO decorated ZnO NRs have been grown on Fe alloy substrates and, acting as cold electron emitters, display excellent field emission performance with the turn-on field Eto as low as 1.63 V/mm and the threshold field Ethr down to 3.12 V/mm. In the case of the bare ZnO NRs, the F-N plot exhibits linear behavior only at very high electric fields, for GO sheets decorated on ZnO NRs, however, the plot is a straight line for nearly all measured fields. The effective work function has been calculated from the slope for the decorated GO sheets with a value below 1.5 eV. Both the experimental results and theoretic analysis indicate that the high efficient electron emission from GO decorated ZnO NRs is attributed to (1) a schottky contacts of Fe-ZnO and matched Fermi level of ZnO-GO interface, (2) the formation of some nanometerscale sharp protrusions in the GO sheets and (3) the C-O-C oxygen functional groups in the GO. This work provides some prospects for the development of practical electron sources and advanced optronic devices based on GO field emitters. Acknowledgements This work is supported by the National Natural Science Foundations of China (Grant No. 11447116), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2016JQ5037), Special Program for Scientific Research of Shaanxi Educational Committee (Grant No. 16JK1601), Doctoral Scientific Research Startup Foundation of Xi'an Shiyou University (Grant No. 2016BS12) and Scientific Research Innovation Team Construction Plan of XSYU (Grant No. 2014KYCXTD02). References [1] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Crystallite size dependence of band gap energy for electrodeposited ZnO grown at different temperatures, Sol. Energy Mater. Sol. Cells 90 (2006) 2356e2361. [2] T. Ichikawa, S. Shiratori, Fabrication and evaluation of ZnO nanorods by liquidphase deposition, Inorg. Chem. 50 (2011) 999e1004. [3] S.W. Cho, Y.T. Kim, W.H. Shim, S.Y. Park, K.D. Kim, H.O. Seo, N.K. Dey, J.H. Lim, Y. Jeong, K.H. Lee, Y.D. Kim, D.C. Lim, Influence of surface roughness of aluminum-doped zinc oxide buffer layers on the performance of inverted organic solar cells, Appl. Phys. Lett. 98 (2011) 023102. [4] J.J. Ding, M.Q. Wang, Physical deoxygenation of graphene oxide paper surface and facile in situ synthesis of graphene based ZnO films, Appl. Phys. Lett. 105 (2014) 233106. [5] G.H. Zhang, L. Wei, Y.X. Chen, L.M. Mei, J. Jiao, Field emission property of ZnO nanoneedle arrays with different morphology, Mater. Lett. 96 (2013) 131e134. [6] H.M. Chiu, H.J. Tsai, W.K. Hsu, J.M. Wu, Experimental and computational insights in the growth of gallium doped zinc oxide nanostructures with superior field emission properties, CrystEngComm 15 (2013) 5764e5775.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Field emission of graphene oxide decorated ZnO nanorods grown on Fe alloy substrates Jijun Ding a, b, *, Haixia Chen a, Li Ma b, Haiwei Fu a, Xiaojun Wang b, ** a b
College of Science, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2017 Received in revised form 19 September 2017 Accepted 20 September 2017 Available online 21 September 2017
Graphene oxides (GO) are decorated on the top surface of ZnO nanorods (NRs) grown on Fe alloy substrates for efficient field emission. The GO decorated ZnO NRs acting as cold electron emitters exhibit excellent field emission performance with the turn-on field Eto as low as 1.63 V/mm and the threshold field Ethr down to 3.12 V/mm. ZnO NRs grown on the alloy substrates have a low interfacial resistance and intend to enhance electrical conduction. A schottky contact of Fe-ZnO and matched Fermi levels of ZnO-GO interface contribute to the enhanced current emission efficiency. Besides, some nanometer-scaled sharp protrusions have been formed in the GO sheets, and GO itself owns abundant C-O-C oxygen functional groups that also help to improve the field emission current. A straight line Fowler-Nordheim plot of the field emission current from the emitter is obtained and the effective work function for the decorated GO sheets is calculated from the slope with a value below 1.5 eV. Finally, field emission mechanism of the GO decorated ZnO NRs has been proposed. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. © 2017 Elsevier B.V. All rights reserved.
Keywords: Graphene oxides ZnO nanorods Field emission Emission mechanism
1. Introduction ZnO is an ideal material for optoelectronic devices due to its wide band gap (~3.37 eV) and large exciton binding energy (~60 meV) [1,2]. It can be applied to ultraviolet lasers, transparent conductive contacts, laser diodes, solar cells, thin film transistors and emitters [3,4]. One-dimension (1D) nanostructures such as ZnO nanorods (NRs), ZnO nanowires (NWs), and ZnO nanocones have attracted considerable interest due to their novel properties and applications. Among them, field emission properties of various 1D ZnO nanostructures have been explored extensively. Zhang et al. [5] reported that field emission properties have a close relationship with the surface morphology of ZnO nanoneedle arrays. Chiu et al. [6] concluded that the turn-on field of ZnO NWs can be significantly decreased after Ga doping but is insensitive to the contents of Ga doping. Li et al. [7] fabricated the ZnO NWs by tuning density with
* Corresponding author. College of Science, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China. ** Corresponding author. E-mail addresses: [email protected] (J. Ding), [email protected] (X. Wang). https://doi.org/10.1016/j.jallcom.2017.09.216 0925-8388/© 2017 Elsevier B.V. All rights reserved.
the thickness of the Al film and displayed excellent field emission performance. To further improve the field emission performance from nanostructured ZnO emitters, different methods are reported, such as doping with metals [8,9], optimizing their shape and aspect ratio [10], decorating with Au nanoparticles [11], exposing to UV [12] and coating surface with low work function materials for low turn on field [13]. On the other hand, composite nanostructures of ZnO and graphene or graphene oxide (GO), owing to excellent field emission, have already been highlighted [14,15]. Graphene as a twodimensional macromolecular sheet of carbon atoms has superior electrical conductivity and mechanical properties [16], making it an excellent electron-transport material [17]. Zhang et al. [18] investigated electronic structures and field emission properties of hybrid graphene-ZnO by density functional theory and showed that the graphene-ZnO is a promising candidate for a field emission electron source. Hwang et al. [19] reported that ZnO NWs/graphene hybrids exhibit excellent field emission properties owing to the mechanical flexibility, transparency and low contact barrier. Recently, ZnO NW tips are coated by graphene dispersion to increase surface volume ratio for further improving field emission [20], however, graphene did not inherit the nanotip geometry since
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graphene is in debris segment after spin coating and plasma etching. Interestingly, Ye et al. [21] found that high density protrusions can be localized in a large area and thus enhance the electric field when GO sheets are coated on the top surface of the Ni array nanotips. Until now, there have been few attempts of surface decorating GO on alloy substrate semiconductor ZnO NRs and concerning their electron emission characteristics and the emission mechanisms associated with the GO's low dimensionality. Herein, we demonstrate a novel nanostructures that may find potential applications in high performance field emitters considering a synergistic effect of alloy substrate (enormous free electrons), ZnO NRs (high aspect ratio) and GO (numerous sharp edges and oxygen functional groups). In addition, the semiconductor nanostructures deposited on alloy substrates provide important technological merits such as the low interfacial resistance and intend to enhance electrical conduction in field emitters, and thus improve current emission efficiency. In this paper, based on GO with unique chemical structures that are easier to form excellent contact with semiconductor ZnO than graphene, combining the superior properties of GO with high aspect ratio of 1D ZnO NRs, GO decorated ZnO NRs have been grown on Fe alloy substrates. Unlike graphene being coated on the top of the emitter and shaped as the emission tips in previous literature, in our work, thin even GO sheets with abundant C-O-C oxygen functional groups are designed to decorate on ZnO NRs. Results indicate that this not only provides another channel for efficient improving field emission due to the matched Fermi levels between ZnO and GO interface, but also further decreases the potential barrier during the field emission process due to a low effective work function of the decorated GO sheets. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. 2. Experimental section Metal alloy substrate containing mainly iron (99 at.%) are cut, ground and polished, then ultrasonically clean with acetone and alcohol in sequence for 15 min. Subsequently, argon plasma clean for 10 min before growing ZnO NRs. ZnO NRs coated with GO sheets are fabricated by the following procedures. Firstly, ZnO seed layers are deposited onto the cleaned metal Fe alloy substrates by the conventional sol-gel and spin-coating technique and annealed at 350 C for 1 h in air. Secondly, ZnO NRs are synthesized by chemical bath deposition (CBD) method. The ZnO seeds are immersed in a 100 ml mixed aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 25 mM) and hexamethylenetetr-amine (C6H12N6, 25 mM) at 95 C for 6 h, and the surface are polished with the seeds side facing downward. After growth, the samples are washed by deionized water and are dried by nitrogen gas. The similarly fabricated processes about ZnO NRs have been reported elsewhere [22]. In addition, the GO is prepared from natural graphite (Wodetai Ltd. Co., Beijing, China, 99.9%) by a classical Hummers method with some modification [23]. The GO brilliant-yellow mixture is filtered and washed with 10 wt.% HCl aqueous solution (1000 ml) to remove metal ions and wash repeatedly with H2O to remove the acid until the pH of the filtrate is neutral. The resulting GO slurry is dried in a vacuum oven at 60 C. Typically, 50 mg of the as-prepared GO powers are dispersed into 100 ml of water under mild ultrasound for 1 h. Then, the GO solvents are centrifuged with 4800 rpm/min to obtain even and stabilized GO with transparent yellow-brown suspension. Finally, GO solvents with good dispersions are used directly as electrolyte. The coated GO sheets on the down-layer ZnO NRs is achieved by electrodeposition. Fe metal alloy substrate with as grown ZnO NRs is acted as working electrode and
539
platinum plate is cathode that is kept 10 mm away from the counter electrode, a constant current of 0.10 mA is applied for 20 min to coat GO on the down-layer ZnO NRs. The sample is dried in nitrogen before characterization of various properties. X-ray diffraction (XRD) patterns of the bare ZnO NRs and GO decorated ZnO NRs are studied using a D/Max-2400 X-ray diffractometer. The scanning electron microscopy (SEM) images of samples are characterized by a FEI Quanta 250 scanning electron microscope. Raman spectra are investigated using a JY LabRAM HR800 laser Raman spectrometer from 200 to 3000 cm1 at room temperature. The 633 nm line of the laser is used as the excitation source. The current density-field strength (J-E) characteristics are estimated under base pressure 105 Pa at RT using a computercontrolled power source with amperometer (Keithley, 248). A glass plate with ITO is used as the anode and the samples are served as the cathode. The distance between the cathode and the anode is kept 300 mm, which is adjusted with insulating polyethylene film with a high breakdown voltage and certain thickness before the measurements. All spectra are measured at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of bare ZnO NRs and GO decorated ZnO NRs grown on Fe alloy substrates. ZnO (100), (002), (101), (102) and (103) diffraction peaks and Fe alloy substrate phase are observed in the XRD patterns. As GO sheets are decorated on ZnO NRs, ZnO (002) diffraction peak intensity increases. For comparison, ZnO and Fe diffraction peaks from JCPDS standard cards are drawn on the bottom of Fig. 1. In addition, as prepared GO has a diffraction peak centered at 2q ¼ 10.8 corresponding to a GO (002) phase. It is theoretically expected to be observed in the XRD pattern. However, the GO peaks are not clearly seen in the XRD patterns due to the small amount of GO in the sample compared with ZnO, which is similar with the previous report [24]. Therefore, it can be seen that the diffraction peaks are mainly from the ZnO and Fe alloy substrate phase in the GO decorated ZnO NRs. According to the Bragg formula: l ¼ 2dhklsinq, where dhkl denotes the crystalline plane distance for indices (hkl). It is found that all d(002) values, which are 0.2596 and 0.2590 nm for bare ZnO NRs and GO decorated ZnO NRs, respectively, are smaller than that of standard ZnO d0 value of 0.2603 nm. The lattice constants c of (002)
Fig. 1. XRD patterns of bare ZnO NRs (middle) and GO decorated ZnO NRs (top) grown on Fe alloy substrates. The ZnO and Fe diffraction peaks from JCPDS standard cards are drawn on the bottom by black and magenta lines, respectively.
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peak can also be calculated by the following formula [25]:
dhkl
2
" #1 4 h2 þ k2 þ hk l2 ¼ þ 2 : c 3a2
(1)
where both parameters a and c are the lattice constants. According to Eq. (1), the lattice constant c is equal to 2d for the (002) diffraction peak. Fig. 2 presents SEM images of bare ZnO NRs (a-b), GO decorated ZnO NRs (c-e) with different magnifications. As shown in Fig. 2aeb, the ZnO NRs with hexagonal structures are presented in the SEM image of bare ZnO NRs. The ZnO NRs are approximately same in tall and the diameter of ZnO NRs is even. After GO decorating, the ZnO NRs are almost fully covered with thin transparent GO layers, as Fig. 2cee depicts. Based on the SEM observation shown in Fig. 2e, the corresponding diameter distribution of ZnO NRs and average
area of GO sheets is shown in Fig. 2f. The average diameter of ZnO NRs and the average area of GO sheets are about 80 nm and 3.6 mm2 calculated using the particle size statistics distribution software. In order to further investigate the crystalline quality, structural disorder, and defects in samples, Raman spectra of samples are characterized, which is generally considered to be the most powerful nondestructive technique [26]. As shown in Fig. 3, a peak centered at 553 cm1 associating with oxygen deficiency in the bare ZnO NRs is observed, which is also reported by Xu et al. [27]. As GO sheets are decorated on ZnO NRs, the intensity of the peak at 553 cm1 decreases. At the same time, both sharp peaks at 1327.5 and 1591.9 cm1 corresponding to the D and G peaks of GO sheets are observed, respectively. The D peak is due to the presence of structural disorders in GO sheets. The G peak is attributed to the optical E2g phonons at the Brillouin zone center, whereas the ratio of the intensity of the G-band to the D-band is related to the inplane crystallite size [28]. Besides, the intensity ratio of IG/ID is
Fig. 2. SEM images of bare ZnO NRs (aeb) and GO decorated ZnO NRs (cee) with different magnifications. The ZnO NRs with hexagonal structures are presented in the SEM image of bare ZnO NRs. After GO decorating, the ZnO NRs are almost fully covered with thin transparent GO layers. The average diameter distribution of ZnO NRs (columnar bar) and the average area of GO sheets ((dotted line)) are shown in (f).
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! 3 J Ab2 BF2 1 ln 2 ¼ ln , ; F b E E
541
(3)
3
6:83 103 F2 cm1 : K
b¼
(4)
where J is in the unit of mA/cm2, E in V/mm, and F the work function of the emitter, that is 5.3 eV for ZnO. A ¼ 1:54 106 A,eV,V 2 , 3 B ¼ 6:83 109 eV 2 ,V,m1 , and b is the field emission enhancement factor that is introduced to quantify the enhancement degree of any tip over a flat surface, i.e., b represents the true value of the electric field at the tip compared to its average macroscopic value [31]. Fig. 5 illustrates F-N plots of bare ZnO NRs and GO decorated ZnO NRs grown on Fe alloy substrates. The F-N plot of the field emission current from the bare ZnO NRs displays a nonlinear relation between ln(J/E2) and 1/E. It is only at very high electric fields that the F-N plot exhibits linear behavior. This nonlinearity can be ascribed to the nonuniform geometries of the emitters [21]. The estimated field enhancement factor b is 1268 cm1 calculated from the slope K of ln(J/E2)-1/E plots. As GO sheets are decorated on ZnO NRs, the plot is a straight line for nearly all measured fields, indicating that the field emission process is a barrier tunneling, i.e., a quantum mechanical process [32]. It is assumed that the local b of the sharp GO protrusions is similar to that of the ZnO NRs. Using the slope, we calculate an effective work function for the decorated GO sheets with a value below 1.5 eV. The reduction of the work function of GO sheets may be due to the sharp bend at the protrusions, and similar result is reported in the GO coated Ni nanoarrays and an effective work function of GO is below 2 eV [21]. All these results indicate that the field emission of ZnO NRs is improved largely by surface decorating GO sheets. Fig. 6 depicts the field emission current stability of GO decorated ZnO NRs for over 2 h at the distance of 300 mm from sample surface to anode under an applied constant voltage of 1425 V. The initial current density is at 0.70 mA/cm2 and the mean value Jmean is about 0.69 mA/cm2. We evaluate the field emission stability of the GO =
Fig. 3. Raman spectra of bare ZnO NRs (lower line) and GO decorated ZnO NRs (upper line) grown on Fe alloy substrates. Only a peak centered at 553 cm1 associating with oxygen deficiency is observed in the bare ZnO NRs. As GO sheets are decorated on ZnO NRs, the intensity of the peak at 553 cm1 decreases. At the same time, both sharp peaks at 1327.5 and 1591.9 cm1 corresponding to the D and G peaks of GO sheets are observed, respectively.
widely used to characterize the defect quantity in graphene and a low ratio indicates a great disorder arising from structural defects [29], which is consistent with XRD results. Fig. 4 displays field emission J-E characteristics of ZnO NRs and GO decorated ZnO NRs grown on Fe alloy substrates. In order to better compare with GO decorated ZnO NRs, the current density intensity of ZnO NRs is expanded 12 times. The turn-on field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. The Eto value of ZnO NRs and GO decorated ZnO NRs are 3.37 and 1.63 V/mm, and Ethr are 14.00 and 3.12 V/mm, respectively. The field emission J-E characteristics can be expressed by a simplified Fowler-Nordheim (F-N) equation [30]:
J¼
Ab2 E2
F
exp
! 3 BF2 : bE
(2)
The formula can be changed as following:
Fig. 4. Field emission J-E characteristics of bare ZnO NRs (solid squared line and its 12times enlargement for better comparison) and GO decorated ZnO NRs grown on Fe alloy substrates (dotted line). The turn-on field Eto and the threshold field Ethr are defined as the field required at a current density of 1.0 mA/cm2 and 0.1 mA/cm2, respectively. Both values largely decrease due to the surface decorating of GO sheets, indicating the improvement of field emission characteristics.
Fig. 5. F-N characteristics of bare ZnO NRs (solid squared line) and GO decorated ZnO NRs (dotted line) deposited on Fe alloy substrates. In the case of the bare ZnO NRs, it is only at very high electric fields that the F-N plot exhibits linear behavior. As GO sheets are decorated on ZnO NRs, the plot becomes a straight line for nearly all measured fields. It is assumed that the local b of the sharp GO protrusions is similar to that of the ZnO NRs. Using the straight line slope, we calculate the effective work function for the decorated GO sheets with a value below 1.5 eV, indicating that the potential barrier of the electrons decreases, making the emission efficient.
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Fig. 6. Field emission current stability characteristics of GO decorated ZnO NRs for over 2 h under an applied constant voltage of 1425 V. The wine solid line shows the linear fitted result and the Jdrop is 1.79% during the test period. The inset is a uniform surface emission image of GO decorated ZnO NRs. The diameter of the emission image is about 0.75 cm.
decorated ZnO NRs using a parameter Jdrop. It is calculated from (Jfirst-Jlast)/Jfirst, where Jfirst and Jlast are the first and the last J values, respectively. The Jdrop value of the GO decorated ZnO NRs is as low as 1.79%. The light drop of the emitted current is due to fieldinduced gas molecule adsorption and desorption from sample surface in the vacuum chamber. There are two other reasons that affect the stability of field emission including both vacuum condition and migration effects for different emission sites. Combined with low both turn on field and the threshold field. GO decorated ZnO NRs produce excellent field emitters, which is critical for practically field emission applications. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. The inset presents a uniform surface emission image of GO decorated ZnO NRs. The diameter of the emission image is about 0.75 cm. The mechanism of electron emission can be explained as following. ZnO NRs without coating GO have higher turn-on and the threshold electric field, which is due to the significant screening of the electric field as a consequence of the areal density of ZnO NRs. This decreases field emission current. After surface decorating
GO sheets, their field emission is improved largely. Fig. 7a presents the schematic drawing of the GO decorated ZnO NRs structures and field emission measurement at an external electric field. A glass plate with ITO is used as the anode and the samples are served as the cathode. The distance between the cathode and the anode is kept 300 mm. We believe that field emission of the GO decorated ZnO NRs structures can take place in the following three possible ways. (1) As can been seen from Fig. 7a, the electrons are transport from the bottom Fe alloy substrates to ZnO NRs and then perpendicular tunneling emission on the top surface of GO. Fig. 7b shows schematic diagram of the corresponding band scheme in the GO decorated ZnO NRs grown on Fe substrate. In our work, ZnO NRs grown on Fe alloy substrates have a low interfacial resistance and intend to enhance electrical conduction. Meanwhile, it is known that when a metal is brought in contact with semiconductor, it induces band bending due to the equilibrium of Fermi level [33]. According to previous conclusion [34,35], a schottky contact is formed at the interface region and a downward band bending is expected since the work function of metal Fe (4.5 eV) is smaller than semiconductor ZnO (5.3 eV). So electrons are easily transferred from the Fermi level of Fe alloy substrates to the top ZnO NRs. At a very low electric field, these electrons are excited from the Fe Fermi level to the ZnO conduction band. The Fermi levels between the ZnO and GO interface are matched through the generation of a space charge region, such as the p-n junction, which have been confirmed in GO coated multiple face contact junctions ZnO NRs [15]. There have also been reported that p-n junction is formed between GO and other semiconductor interface, for example in the GO/TiO2 composites [36]. Due to the electron affinity of GO is lower than that of ZnO [37], which drives the excited electrons from the ZnO conduction band to the surface of GO through ZnO-C bonding interactions. At an external electric field, these electron transport processes are accelerated. Besides, as above mentioned, according to the calculated results based on F-N plots, the effective work function for the decorated GO sheets is below 1.5 eV. Both the potential barrier height and width for the electron emission are further reduced. This further decreases the potential barrier of the electrons and makes the emission efficient. So electrons can easily tunnel through the full barrier width, causing a larger field emission current in GO decorated ZnO NRs. (2) Compared with bare ZnO NRs, both the number of macroscopic emission sites decreases and field screening effect gets weaker in GO decorated ZnO NRs. However, some nanometer-scale sharp protrusions are formed in the GO sheets as they are decorated on the top surface of ZnO NRs.
Fig. 7. (a) Schematic diagram of the GO decorated ZnO NRs structures and field emission measurement at an external electric field. The electrons are transported from the bottom Fe alloy substrates to ZnO NRs and realized perpendicular tunneling emission on the top surface of GO. A glass plate with ITO is used as the anode and the samples are served as the cathode. (b) Schematic diagram of the energy band scheme and electron transport in the GO decorated ZnO NRs on Fe substrate, and electron tunneling through the full barrier width. A schottky contact and a downward band bending are formed at the interface region between Fe and ZnO. The Fermi levels between the ZnO and GO interface are matched through the generation of a space charge region, such as the p-n junction. (EVAC: vacuum level, EV: the top of the valence band, EC: the bottom of the conduction band, EF: Fermi level, J(E): the emission current density, F: work function).
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These high density protrusions enhance the local electric field of the emitter tips and cause the perpendicular emission from 2D GO surfaces associated with its low dimensionality. The perpendicular emission has been confirmed in the individual graphene nanoribbons with high emission density [38]. (3) There is another enhanced origin of field emission needed to be considered. GO itself owns a large number of C-O-C oxygen functional groups existing in its 2D surface and edge structures, according to previous report [39], during the field emission process, the presence of C-O-C groups at its edge structures significantly reduces the barrier for electron emission into vacuum. In summary, the electron emission from GO decorated ZnO NRs is attributed to a schottky contact of Fe-ZnO and matched Fermi levels of ZnO-GO interface, the formation of some nanometer-scale sharp protrusions in the GO sheets and the C-O-C oxygen functional groups in the GO. This work may help the development of the practical electron sources and advanced optronic devices based on GO field emitters. 4. Conclusions In conclusion, GO decorated ZnO NRs have been grown on Fe alloy substrates and, acting as cold electron emitters, display excellent field emission performance with the turn-on field Eto as low as 1.63 V/mm and the threshold field Ethr down to 3.12 V/mm. In the case of the bare ZnO NRs, the F-N plot exhibits linear behavior only at very high electric fields, for GO sheets decorated on ZnO NRs, however, the plot is a straight line for nearly all measured fields. The effective work function has been calculated from the slope for the decorated GO sheets with a value below 1.5 eV. Both the experimental results and theoretic analysis indicate that the high efficient electron emission from GO decorated ZnO NRs is attributed to (1) a schottky contacts of Fe-ZnO and matched Fermi level of ZnO-GO interface, (2) the formation of some nanometerscale sharp protrusions in the GO sheets and (3) the C-O-C oxygen functional groups in the GO. This work provides some prospects for the development of practical electron sources and advanced optronic devices based on GO field emitters. Acknowledgements This work is supported by the National Natural Science Foundations of China (Grant No. 11447116), Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2016JQ5037), Special Program for Scientific Research of Shaanxi Educational Committee (Grant No. 16JK1601), Doctoral Scientific Research Startup Foundation of Xi'an Shiyou University (Grant No. 2016BS12) and Scientific Research Innovation Team Construction Plan of XSYU (Grant No. 2014KYCXTD02). References [1] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Crystallite size dependence of band gap energy for electrodeposited ZnO grown at different temperatures, Sol. Energy Mater. Sol. Cells 90 (2006) 2356e2361. [2] T. Ichikawa, S. Shiratori, Fabrication and evaluation of ZnO nanorods by liquidphase deposition, Inorg. Chem. 50 (2011) 999e1004. [3] S.W. Cho, Y.T. Kim, W.H. Shim, S.Y. Park, K.D. Kim, H.O. Seo, N.K. Dey, J.H. Lim, Y. Jeong, K.H. Lee, Y.D. Kim, D.C. Lim, Influence of surface roughness of aluminum-doped zinc oxide buffer layers on the performance of inverted organic solar cells, Appl. Phys. Lett. 98 (2011) 023102. [4] J.J. Ding, M.Q. Wang, Physical deoxygenation of graphene oxide paper surface and facile in situ synthesis of graphene based ZnO films, Appl. Phys. Lett. 105 (2014) 233106. [5] G.H. Zhang, L. Wei, Y.X. Chen, L.M. Mei, J. Jiao, Field emission property of ZnO nanoneedle arrays with different morphology, Mater. Lett. 96 (2013) 131e134. [6] H.M. Chiu, H.J. Tsai, W.K. Hsu, J.M. Wu, Experimental and computational insights in the growth of gallium doped zinc oxide nanostructures with superior field emission properties, CrystEngComm 15 (2013) 5764e5775.
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